The present invention relates to compounds, pharmaceutical compositions, and methods for treating several conditions with prodrugs for phenylacetate as therapeutic agents.
1. Field of the Invention
Phenylalkanoic acids are known therapeutic agents for a variety of disorders. Phenylacetate is used for the treatment of nitrogen metabolism disorders, beta-hemoglobinopathies, anemia and cancer. Various phenylalkenoic acids can be used in the treatment of the same disorders. The prodrugs disclosed in the present invention are useful therapeutic agents for a number of disorders, and possess some advantages over the forms of the drugs administered in the prior art.
Nitrogen Metabolism Disorders
In a healthy person, the potentially toxic nitrogenous compounds which accumulate as the body degrades proteins are synthesized into urea which is rapidly excreted into the urine. However, for those who suffer kidney failures liver failure or inborn errors of urea synthesis, this pathway is defective. The accumulation of nitrogenous compounds resulting from such a blockage leads to considerable morbidity and mortality.
In the case of an inborn error of urea synthesis, the major metabolic abnormality is the inability of the body to convert waste nitrogen into urea. As a consequence, various nitrogenous metabolites accumulate in the body, the most toxic being ammonium, although other materials, such as glutamine and alanine also increase.
Previous therapeutic approaches for treating patients with urea cycle enzymopathies (as well as other nitrogen accumulation diseases cited earlier) have been designed to reduce the requirement for urea synthesis by quantitative and qualitative manipulation of dietary protein, amino acids and/or their nitrogen free analogues. Generally speaking, however, the mortality of inborn errors of the urea-cycle remained high and success was measured in terms of increased survival time. Thus, for example, even with the above-cited therapeutic approaches, children with the neonatal form of these diseases rarely survive past one year of age (Maestri, et al., The Journal of Pediatrics, Vol. 119, No. 6, 923-928 (1991)).
2. Description of Related Art
A more recent approach to remedy this pervasive problem is described in U.S. Pat. No. 4,284,647 to Saul W. Brusilow, wherein benzoic acid, phenylacetic acid, or the salts thereof, convert the waste nitrogen into amino acid acylation products which the body can successfully excrete as urinary nitrogen. More specifically, the patent teaches that phenylacetate reacts with the nitrogen to form phenylacetylglutamine which is subsequently excreted by the body. Since such a reaction is in no way dependent on the urea synthesis or excretion, it is an effective treatment for those suffering from nitrogen accumulation diseases. See also "Treatment of Inborn Errors of Urea Synthesis," New England Journal of Medicine, 306; 1387-1392 (1982).
U.S. Pat. No. 4,457,942, also to Saul W. Brusilow, discloses that even-numbered phenylalkanoic acids can be advantageously used for the treatment of nitrogen accumulation diseases.
When administered to humans, even numbered phenylalkanoic acids, such as phenylbutyrate, can be broken down by beta-oxidation, two carbon atoms at a time, to eventually yield phenylacetate which, as described above, has been found useful for removing waste nitrogen from the blood stream. The administration of even numbered phenylalkanoic acids such as phenylbutyrate has the advantage that the higher molecular weight compounds do not have the offensive odor-which phenylbutyrate has.
The above treatments, although effective, have a substantial disadvantage. The dose of sodium phenylbutyrate for an adult with a urea cycle disorder is 20 grams/day. This requires that the patient take forty (40) tablets of 0.5 grams each, per day. Problems of patient compliance arise when such large daily doses are required. The administration of sodium phenylbutyrate has a second disadvantage to many patients--patients who should restrict their daily dose of sodium. The above daily dose of sodium phenylbutyrate provides 2.5 gm of sodium per day, every day (it is recommended that adults consume less than 2.4 grams/day total sodium).
The substitution in therapy of phenylacetate or phenylbutyrate, by the compounds of the present invention, provides the therapeutic compound in a more convenient dosage form. In addition, the compounds of the present invention may eliminate the peaks and valleys in drug levels since the breakdown of these higher molecular weight compounds by beta-oxidation is a gradual process. In addition, the Na component of the prior art is replaced with glycerol, which is a normal product of metabolism.
Cancer
Phenylbutyrate and phenylacetate are being investigated as a treatment for various malignant diseases. The exact mechanism by which this therapy causes improvement in the patient is not entirely clear.
It has been observed that primary central nervous system tumors are reminiscent of immature brain, and the immature brain is known to be more vulnerable to damage by phenylacetate than the mature brain (as is observed in phenylketonuria). Sodium phenylacetate appears to promote the differentiation of cultured human glioblastoma cell lines with reduced expression of malignant phenotype.
Systemic treatment of rats bearing intracranial gliomas with phenylacetate resulted in significant tumor suppression with no apparent toxicity to the host. Early clinical results suggest that phenylacetate may become an important tool in the management of certain tumors in light of its demonstrated efficacy, and lack of toxicity (Samid et al., Cancer Research, 54, 891-895, 1994, and Cinatl et al., Cancer Letters, 70, 15-24, 1993).
A similar theory may be applied in treating prostate cancer with phenylacetate. The phenylacetate is thought to act as a differentiation inducer of leukemic and other less differentiated tumor cells, such as hormone refractory prostate cancer.
Cultured cells of androgen dependent prostate cell lines with sodium phenylacetate show inhibition of cell proliferation. In addition, such cells show reversion to non-malignant phenotype by in vivo and in vitro assessments (Samid et al., The Journal of Clinical Investigation, Vol. 19, 2288-2295, 1993).
Phenylacetate may exert an anti-tumor affect by another mechanism. Glutamine is the major nitrogen source for nucleic acid and protein synthesis, and substrate for energy in rapidly dividing normal and tumor cells. Compared to normal tissue, most tumors, due to decreased synthesis of glutamine along with accelerated utilization and catabolism, operate at limiting levels of glutamine availability and consequently are sensitive to further glutamine depletion. In the body, phenylacetate conjugates with glutamine, with subsequent renal excretion of phenylacetylglutamine. This pathway is the reason that phenylacetate administration is useful in the treatment of nitrogen accumulation diseases. Because phenylacetate removes glutamine, administration of phenylacetate may limit the growth rate of rapidly dividing cells such as tumor cells.
By one or more of the above mechanisms, phenylacetate causes a decrease in tumor characteristics of a variety of tumor cells. Because of its known non-toxicity, phenylacetate is a promising therapeutic agent, either alone or in combination with other anti-tumor agents.
Hemoglobinopathies Sodium phenylbutyrate is thought to cause improvement in certain .beta.-hemoglobinopathies because the sodium phenylbutyrate induces the expression of fetal hemoglobin. Thus the absent or aberrant .beta.-hemoglobin is substituted with fetal hemoglobin.
Numerous agents which induce the expression of fetal hemoglobin have been used to treat sickle cell anemia and .beta.-thalassemias. Some of the agents which increase the production of fetal hemoglobin however, have serious side effects that are not consistent with their use as long term therapeutic agents. However, sodium phenylacetate and sodium phenylbutyrate have been previously used to treat urea cycle disorders and are known to be very well tolerated and free of adverse reactions in clinical use. Preliminary clinical studies of patients with beta thalassemia indicate that treatment with sodium phenylbutyrate results in a response in many patients. The response is particularly good in patients with relatively high erythropoietin levels. Thus, combination therapy of the phenylbutyrate and erythropoietin may be effective. Hydroxyurea given orally has also been shown to increase hemoglobin levels in some thalassemia patients. Clinical studies of thalassemia patients treated with a combination of hydroxyurea and sodium phenylbutyrate has produced increased hemoglobin levels in some patients.